Document Type
Dissertation
Date of Award
5-31-2022
Degree Name
Doctor of Philosophy in Mechanical Engineering - (Ph.D.)
Department
Mechanical and Industrial Engineering
First Advisor
Pushpendra Singh
Second Advisor
Ian Sanford Fischer
Third Advisor
I. Joga Rao
Fourth Advisor
Zhiming Ji
Fifth Advisor
Dibakar Datta
Sixth Advisor
Alex Pillapakkam
Abstract
This dissertation aims to study the flow induced in a water body due to the presence of a solute or heat source on its surface. The flow arises because of a change in the local solute concentration or the temperature, which modify the local surface tension, giving rise to a surface tension gradient that drives the flow. These surface tension-driven flows are referred to as Marangoni flows. The following three induced-flows are investigated experimentally and theoretically: (i) Flow induced by a solute source present on a waterbody; (ii) Flow induced due to the impingement of a freshwater drop on a saltwater body; and (iii) Flow induced by the motion of a three-phase contact line. The results show that the presence of solutocapillary forces in the first two cases strongly influences the transport properties at the air-water interface. In the third case, the thermocapillary forces affect the motion of the three-phase contact line.
A study of these Marangoni flows is important because they induce mixing in a thin layer near the air-water interface, which enhances the exchange of gases across the air-water interface. According to the analytic solution, the flow induced by a solute source (or a solute sink) on a freshwater body (or saltwater body) is analogous to a doublet flow in the sense that below the source, the flow is directed towards (or away from) the source within a conical region with its vertex at the source and on the surface outside the conical region the flow is away from (or towards) the source. The half cone angle for the induced flow varies with the solute inflow rate. At low solute inflow rates, the angle is about 60°, and increases with increasing solute inflow rate to values greater than 80°. Under the latter conditions, the velocity and concentration boundary layers develop near the surface as the outflow is restricted to a thin axisymmetric wedge-shaped boundary layer region. The presence of sharp velocity gradients in the boundary layer region increases the concentrations of sparingly dissolved gases, such as oxygen and carbon dioxide, in the waterbody, and thus the water quality. The flow induced due to the impingement of a freshwater drop on a saltwater body is similar. On the surface, the flow is away from the impingement point, and below the impingement point it is towards the impingement point. PIV (particle image velocimetry) and PLIF (planar laser-induced fluorescence) data show that the actual flow is far more complex than the analytic solution because of the influence of other factors not accounted for in the analytic model. There is an approximate agreement between the experimental and analytic results, but only for intermediate distances from the source needle. In experiments, the flow changes direction into the water body when the solute-induced surface tension gradient driving the flow becomes comparable to the surface tension gradients on the surface due to factors such as surfactant contamination and temperature gradients. It is shown that the surface flow changes direction at a distance where the solute-induced surface tension gradient reduces to the same order of magnitude as that for the Raleigh-Bernard convection due to evaporative cooling. An expression obtained by setting the solute and RB convection induced surface tension gradients equal approximately estimates the distance over which the surface flow occurs as a function of the source flow rate.
Another focus of the dissertation is on the thermocapillary flow that arises due to a sharp change in the temperature at a three-phase contact line that may occur when a liquid first comes in contact with a solid surface and a contact line forms or when the contact line moves on a solid surface. The primary driving force in these flows is the temperature difference between the surface water and the surface of the solid. The temperature difference arises due to the evaporative cooling at the water surface, which causes it to be cooler than the surrounding air. The solid surface temperature (initially not in contact with water) is closer to the air temperature. So, the solid surface is warmer than the surface water, even when they are under the same ambient conditions. This work presents the measured induced flow velocity near the contact line on a plate that is moved vertically to the undeformed water surface. The results are presented for a range of humidity values and plate speeds. The corresponding temperature gradients on the air-water interface (AWI) due to the motion of the three-phase contact line, measured using an infrared camera, show that there is a negative temperature gradient on the AWI, which derives the flow. Soon after the plate begins to move, the temperature gradient is large and limited to a region near the contact line. As the area over which temperature increase occurs grows, the magnitude of the temperature gradient decreases, and the velocity reaches a steady value.
Recommended Citation
Benouaguef, Islam, "Experimental and theoretical studies of flows driven by soluto- and thermo-capillary forces" (2022). Dissertations. 1916.
https://digitalcommons.njit.edu/dissertations/1916
